The present description relates to a series hybrid electric vehicle (HEV) power-train, and more particularly relates to a method of controlling a series HEV power-train for improving operating efficiency of the power-train.
Series HEV power-trains have been presented, such as in U.S. Pat. No. 6,326,702. The series HEV power-train comprises an internal combustion engine, a first electric machine, and a second electric machine. The engine combusts fuel and converts chemical energy in the fuel to mechanical energy. Then, the engine directly drives the first electric machine, which generates electricity by converting the mechanical energy to the electrical energy. The second electric machine is usually not mechanically coupled to the engine or the first electric machine, but coupled to a vehicle driving wheel for propulsion of the vehicle.
On the other hand, electrically, the first and second electric machines are coupled to each other directly, or through an electric battery. Therefore, the electricity generated by the first electric machine may be directly supplied to the second electric machine or supplied to the electric battery for a later use by the second electric machine. Also, the second electric machine may regenerate electricity in a vehicle deceleration, and that electricity may be stored in the battery for a later use by the second electric machine as well.
From an energy conversion standpoint, the chemical energy in fuel is finally converted to the mechanical energy at the driving wheel, through the couple of energy conversions. For better fuel economy, it is necessary to improve an overall energy conversion efficiency of an entire HEV power-train from the chemical energy in the fuel to the mechanical energy at the wheels.
The inventors herein have recognized that, when the electricity is directly transmitted from the first electric machine to the second electric machine, the two energy conversions from electrical through chemical to electrical energies at the battery can be omitted, and the energy conversion efficiency may be improved for the omission of the two conversions. The conversion efficiency from the electrical to mechanical energies at the second electric machine is simply determined from the load and speed for a power transmission mechanism with a fixed gear ratio, which is common on a series HEV. In this situation, for the better overall efficiency, only the efficiencies at the engine and the first electric machine can be improved.
The '702 patent presents a method of operating an engine at an operating point where it operates most efficiently while outputting desired power. In this method, although the engine may operate with the best available efficiency on the desired power, the first electric machine may operate less efficiently at the operating point which has been determined only in consideration for the engine efficiency. Therefore, there is a room to improve the overall efficiency of a series HEV power-train.